Ng says military assets of a sovereign nation were immune from restraints in another country.

The ruckus over the nine Terrex armoured personnel carriers seized in Hong Kong worsened with China warning Singapore to be cautious with its "remarks and actions" after Prime Minister Lee Hsien Loong asked Hong Kong to return the military vehicles immediately.

After Singapore Defence Minister Ng Eng Hen raised the issue of Singapore's rights to get back the armoured vehicles under international laws, China retorted suggesting Singapore undermined the One-China principle, the bedrock of PRC's claim to the 'renegade' Taiwan.

"It is hoped that all relevant countries, including Singapore, can earnestly respect the one-China policy, which is the fundamental prerequisite for China to develop ties with other countries," Chinese Ministry of Foreign Affairs spokesman Lu Kang said at a briefing in Beijing.

Lu added that the Hong Kong authorities are dealing with the issue under the rules and regulations. Beijing, which was offended by Singapore's stance in the territorial dispute in the South China Sea, criticised Singapore's Defence Minister Ng Eng Hen's remarks that the seizure of the Terrex carriers was in contravention to international laws.

"I hope the relevant parties can be cautious in their remarks and actions ... I want to stress that China hopes other nations, including Singapore, follow the one-China principle. This is the foundation for bilateral ties between China and any other nation," Lu said, according to the South China Morning Post.

Ng had raised the issue of sovereignty of Singapore and asserted that the military assets of a sovereign nation were immune from restraints in another country. "They are protected by sovereign immunity, even though they were being shipped by commercial carriers," Ng said. "They cannot legally be detained or confiscated by other countries," he added.

Meanwhile, Singapore Foreign Affairs Minister Vivian Balakrishnan said in Parliament the city state respected and adhered to the one-China policy.

The tussle over the Terrex armoured carriers, which were seized in Hing Kong in November, came to the fore again after Prime Minister Lee wrote to Hong Kong chief Executive CY Leung, urging him to return the vehicles immediately. While Hong Kong insists that the issue is a matter for Hong Kong Customs to decide in normal course, Singapore has said it has received no formal reasons for the detention for over two months.